Heretofore, in the process of producing semiconductor devices, a gas etching for partially removing a thin-film material is performed for forming a circuit pattern on various thin-film materials constituting a semiconductor circuit and at the same time, removal of deposits using a cleaning gas is performed to remove a thin-film starting material deposited to the reactor during the thin film formation. One of useful etching gases or cleaning gases conventional for the production process of a semiconductor device is octafluoropropane (hereinafter referred to as “FC-218”).
On the other hand, to keep up with recent tendency toward higher performance, smaller size, higher density wiring of electrical or electronic equipment, the circuit patterns are becoming finer and in order to form a higher-precision circuit pattern by etching, use of a high-purity etching gas from which impurities are eliminated as much as possible is demanded. When an etching gas contains an impurity even a very small amount, this may cause generation of a large width line during the formation of a fine pattern and increase of defects in the product having a high density integrated circuit.
Also in the process of removing deposits using a cleaning gas, residual impurities in the production process of a semiconductor device after cleaning must be reduced as much as possible so as to provide a high-purity and high-quality device. For this purpose, a high-purity cleaning gas containing substantially no impurity is demanded.
With respect to the production process of FC-218, for example, a method of electrolytically fluorinating 1-chloropropane (see, U.S. Pat. No. 3,709,800), a method of reacting trifluoropentachloropropane with manganese trifluoride (see, U.S. Pat. No. 2,578,721) and a method of reacting hydrogen fluoride and chlorine with compounds such as propane and propylene (see, U.S. Pat. No. 5,220,083) are known. However, in these methods, a compound containing chlorine is used as a starting material and therefore, a chlorine-containing compound is produced as a by-product and mixed into FC-218 as impurities.
On the other hand, with respect to the method of producing FC-218 using chlorine-free starting materials, for example, a method of electrolytically fluorinating propane is known (see, U.S. Pat. No. 3,840,445). However, these methods are industrially disadvantageous because the apparatus therefor is very complicated and the yield is low.
In addition, a method of fluorinating hexafluoropropene (hereinafter sometimes referred to as “FC-1216”) to produce FC-218 is known. For example, a method of reacting FC-1216 with a fluorine gas under dilution with an inert gas and a reaction product gas, a method of electrolytically fluorinating FC-1216 in hydrogen fluoride (see, JP-B-62-61115) (the term “JP-B” as used herein means an “examined Japanese patent publication”), and a method of reacting at least one high-valence metal fluoride selected from cobalt trifluoride, manganese trifluoride and silver difluoride with FC-1216 (see, JP-B-62-54777) are known.
In this case, to produce FC-1216, for example, a process comprising thermal decomposition of chlorodifluoromethane (hereinafter sometimes referred to as “HCFC-22”) and a process comprising fluorinating perhalogenated chlorofluorocarbon having 3 carbon atoms and then dehalogenating the fluorination product to produce FC-1216 (see, U.S. Pat. No. 5,057,634) are known.
However, also in these methods, a chlorine-containing compound is usually used as a starting material and therefore, the obtained FC-1216 usually contains a chlorine-containing compound as impurities. As a result, FC-218 produced starting from this FC-1216 contains the chlorine-containing compound together with unreacted FC-1216.
Accordingly, these chlorine-containing compound and fluorocarbon impurities such as FC-1216 must be removed from FC-218.
For example, separation of these impurities from FC-218 by distillation or the like is attempted. More specifically, impurities contained in FC-218 can be theoretically removed by distillation if these have a boiling point different from that of FC-218. However, as shown in Table 1 below, chloropentafluoroethane, (hereinafter sometimes referred to as “CFC-115”), FC-1216, dichlorodifluoromethane (hereinafter sometimes referred to as “CFC-12”) and HCFC-22, which are mixed in many cases as impurities, each has a boiling point approximated to the boiling point of FC-218. Therefore, it is very difficult to separate these impurities by distillation and obtain high-purity FC-218.
TABLE 1StructuralBoilingCompound NameFormulaPoint (° C.)Octafluoropropane (FC-218)CF3CF2CF3−36.7Chloropentafluoroethane (CFC-115)CC1F2CF3−38.7Hexafluoropropene (FC-1216)CF3CF═CF2−31Dichlorodifluoromethane (CFC-12)CC12F2−29.8Chlorodifluoromethane (HCFC-22)CHClF2−41
Therefore, a purification method other than the separation by distillation, such as extractive distillation, membrane separation and adsorption separation, is being attempted.
However, the extractive distillation method has a problem in that the equipment costs highly and the process is cumbersome. The membrane separation method has a problem in that an appropriate and practical membrane having properties necessary for separating FC-218 from impurities is not known, and purification to high purity, for example, such that the content of impurities in FC-218 is 1 ppm by mass or less, is difficult.
Also, as shown in Table 2, there is almost no difference in the molecular size (calculated value at stable state structure) between FC-218 and CFC-115 or FC-1216, there is no difference in the boiling point between FC-218 and impurities, and FC-218 and impurities are approximated in the structure and physical properties. Therefore, purification to a high purity by the removal of impurities cannot be attained by an adsorption separation method using a known adsorbent such as activated carbon, silica gel, zeolite (molecular sieves) and molecular sieving carbon (hereinafter referred to as “MSC”).
TABLE 2Molecular SizeCompound Name(calculated value)Octafluoropropane (FC-218)4.9 to 6.1 ÅChloropentafluoroethane (CFC-115)4.3 to 5.6 ÅHexafluoropropene (FC-1216)4.9 to 5.9 Å
Among these, FC-1216 which is one of impurities can be removed by adsorption using activated carbon or MSC, however, chlorine-containing compounds such as CFC-115 cannot be separated.
Accordingly, in conventional purification methods, it is difficult to obtain high-purity FC-218 by reducing the concentration of fluorocarbon impurities including chlorine compounds such as CFC-115 to less than 1 ppm by mass.
As a result of extensive investigations to solve these problems, the present inventors have found that when crude octafluoropropane containing impurities such as chlorine compounds is contacted with an impurity decomposing agent containing an iron oxide and an alkaline earth metal compound and then with an adsorbent, these impurities can be substantially removed with ease.
More specifically, the present inventors have found a purification process of FC-218, where FC-218 containing fluorocarbon impurities such as CFC-115, FC-1216, CFC-12, CFC-13 (chlorotrifluoromethane) and HCFC-22 in a concentration of 10 to 10,000 ppm by mass is contacted with an impurity decomposing agent and further with an adsorbent and thereby these impurities can be reduced to less than 1 ppm by mass. The present invention has been accomplished based on this finding.